Beams with radial or azimuthal polarization attract significant interest due to unique optical properties associated with their inherent symmetry. Such beams enable resolution below the diffraction limit and interact without the undesirable anisotropy produced by linearly polarized light. The large longitudinal electric field component of these beams is attractive for applications, such as particle acceleration.
Several methods have been proposed for the generation of space variant polarized beams, including birefringence manipulation with liquid crystals and polarization selection inside a laser resonator. However, the low damage threshold restricts the application of liquid crystal based beam converters. Alternatively, it has been demonstrated that the polarization can be manipulated by the form birefringence associated with the sub-wavelength gratings. It has been also observed that the space variant phase produced by these converters can create a polarization vortex, i.e. the beam with the orbital angular momentum, where its sign depends on the handedness of incident circular polarization. Photolithography, which is usually used for fabrication of such structures, has a limiting resolution that restricts the wavelength of operation to the far infrared.
A European patent EP0764858 describes an arrangement, which converts an input light beam into an output beam of light which is linearly polarised in the radial direction essentially over its entire cross-section. The polarisation direction of the input light beam is rotated and is not selected. The arrangement contains a raster, segment or facetted device with more than 4 half-wave plates (4i) whose preferred direction (2i) is aligned so that each half-wave plate deflects the polarisation direction of the linearly polarised light passing through it along a radius (3i) intersecting the half-wave plate on the optical axis.
Patent US2007115551 describes embodiments of an invention, directed to apparatus and methods for converting spatially homogeneously polarized light into spatially inhomogeneously polarized light having a fast axis orientation that varies in a smooth and continuous manner over a pupil aperture. A space-variant waveplate referred to herein as a polarization converter includes an optically transmissive window characterized by a symmetric stress birefringence that provides at least lambda/4 retardance and, more particularly, lambda/2 retardance over an annular region centered about the optical axis of the window. Structural embodiments of the polarization converter include mechanical compression housing and a thermal compression housing. Radially and azimuthally polarized vortex beams including cylindrical vector beams and counter-rotating beams can be generated from uniformly plane polarized input beams propagating through the polarization converter. Low-order polarization vortex beams can be optically combined to produce higher-order scalar vortex beams. Embodiments of the invention are also directed to various optical illumination and imaging systems utilizing the apparatus and methods described herein.
There is a need for polarization converters, which can convert linearly or circularly polarized light beam into radially or azimuthally polarized light beam in the visible light region. The optical element has to be efficient enough in order to use it in industrial and scientific applications. The transmission should be at least 60% or more, which is impossible when using several polarization conversion elements in a row.